A measure of performance of an antenna design is the sidelobe pattern levels relative to a main beam and is measured in negative decibels (−dB). The sidelobes are measured in −dB from peak gain of the main beam down to the peak gain of the sidelobes that are nearest to the main beam in angular position. The desirable decrease in the peak gain of the sidelobe beams relative to the peak gain of the main beam is referred to herein as sidelobe rejection. Desirable high sidelobe rejection rejects unwanted interference and can further enhance imaging in imaging application. Sidelobe rejection is a function of the steered offset angle for both by phasing or delaying. When steered off center, mechanical blockage and electrical signal interference affect the amount of sidelobe rejection. It is desirable, of course, that the sidelobe rejection remain high even when an antenna array is steered off center, which is well suited for antenna tracking applications and interference immunity. Sidelobe rejection is determined in part by the array configuration. Sidelobe rejection can also be measured as a function of beam steering that provides an angular offset from the center Nadir panel boresight. For example, a signal arriving from a far field point arrives at an angle offset and the antenna main beam is mechanically or electrically steered in that direction of the angular offset. The antenna or antenna array can be steered toward the direction of a transceived signal.
The antenna array inherently provides a Nadir panel boresight extending from the center of the antenna. The Nadir panel boresight is the referenced of a null θ=0° angular offset. The boresight can be steered to point at various angles. Mechanically gimbaled steering provides a gimbaled boresight and electronically phased steering provides a delayed boresight. The gimbal boresight and delayed boresight steering have been commonly used to point an antenna array during tracking of a space object. Gimbaled steering requires time delays to electrically align the antenna elements because the mechanical gimbaling introduces small time delays between the various antennas. These time delays have been removed completely using time delays. With gimbal steering, the main beam is no longer aligned to the Nadir panel boresight, but is centered on the gimbaled boresight of an individual reflector, but requires time delays. With phase steering, the main beam is no longer centered on the Nadir panel boresight of an individual reflector, but is centered on delayed boresight, but requires phase shifters or time delays to align all the signals from all of the antennas in the array.
Curious in nature are configurations that provide maximum packing densities. For example, bees make hexagonal hives. Three sided, four sided, and six sided polygons offer maximum density with zero interpolygonal space when these like polygons are positioned juxtaposed. Conventional arrays having a small numbers of elements have been used. Circular antenna elements have long been arranged in arrays. Antenna arrays have also been configured for maximum density of antenna elements. Small antenna arrays are typically arranged in hexagonal or rectangular lattice configurations. Typical arrays are rectangular arrays and the hexagonal arrays. For a small number of elements, the typical array is either a nine-element array or a seven-element array. The nine-element array is arranged in a rectangular pattern. The seven-element array is arranged in a hexagonal pattern. The hexagonal pattern has six outer antenna circumferentially disposed about a center antenna. The rectangular array can be a 3×3 rectangular array. The hexagonal array includes one center antenna circumferentially surrounded by six antennas. Because the antenna elements are circular, there will exist interelemental space between the antenna elements, but the exterior of array generally forms a polygon shape. The rectangular and hexagonal arrays have a minimum amount of interelemental space yet provide an exterior quasi polygonal perimeter offering very high, but slightly less than optimal packing density.
The gain pattern of the small array is a product of the array configuration and the element patterns. The symmetry of these arrangements provides for symmetrical antenna patterns although disadvantageously with high sidelobe levels. Repositioning element positions in a random manner is a well-known technique for reducing sidelobes for large numbers of elements. Decreasing the interelemental space advantageously increases peak gain of the main beam and side lobes. The antennas are typically positioned to touch but not overlap with a desired minimal amount of interelemental space between the perimeters of the reflectors providing an over-all exterior quasipolygonal perimeter. Increasing the interelemental space in an antenna array disadvantageously decreases sidelobe rejection and increases the total physical area required for the same number and size of antennas. The antenna arrays operate under various conditions, but typically have the center main beam projected through and along the center boresight having a plurality of sidelobe beams. Antenna arrays are specifically designed to capture main beam transceived signals in a main beam while disadvantageously capturing unwanted transceived sidelobe signals captured in sidelobe beams. An antenna generates a main beam and several sidelobe beams that are circumferentially disposed about the main beam and extend from near to far from the main beam. Each antenna dish includes a feed horn that operates to provide a power taper from the feed horn to the perimeter of the dish. The power taper radially extending from the feed horn to the perimeter may be, for example, −10 dB.
Antenna steering can be by gimballing the array elements with electrical time delay phase steering or by sole electrical phase steering the array elements. Gimbal steering has been used for single antennas as well as for very large arrays. When Gimbal steering is used, phase steering is also used, preferably using time delays, so that the delaying boresight and the gimbal boresight are in coincident alignment. With gimbaled steering, the difference between the gimbaled offset angle of phased offset angle are initially the same, but in some applications, the phase offset angle is dithered by a very small angular amount. For example, the Nadir panel boresight can be referenced to θ=0°, while the gimbal boresight is moved to θ=10°, and the delayed boresight is dithered between θ=10° and θ=9° degrees providing a 1° degree dither. Phased steering has been used for both planar phased arrays that do not use mechanical gimballing. Conventional planar phase arrays use phase shifters and not time delays for phase steering because the number and costs of required expensive time delays as opposed to the inexpensive phase shifters. Other conventional dish arrays have used time delays for phase steering. Time delays are preferred to eliminate frequency dependencies of the sidelobe rejections, but are expensive for array with a large number of elements.
For example, a 1 GHz signal may be transceived by a 5 m diameter nine-element array. Each element has a −10 dB power taper. The sidelobe levels of the nine element rectangular array are −10 dB below the peak gain of the main beam at a zero offset. The rectangular array of nine reflectors can be mechanically and electrically steered to the center θ=0° with near sidelobes suppressed by −10 dB and with very far sidelobes suppressed by more than −25 dB at 1 GHz. When the frequency is changed from 1 GHz to 0.7 GHz, the main beam and sidelobe peaks remain the same, with the main beam at the θ=0°, but the beams broaden in angular position. There are no frequency dependent grating lobes. The main beam is still positioned on the Nadir planar boresight.
When the offset angle is changed by steering, for example, from θ=0° to θ=10° off the Nadir planar boresight, by both mechanical and electrical steering, the sidelobe rejection remains the same. As such, the nine-element array can be steered mechanically and electrically to a single, frequency-independent, angular position without sidelobe rejection degradation, excepting for the slight loss associate with blockage by mechanical steering. The peak gains of the sidelobes remain approximately the same over frequency and angular position. The angular position of the sidelobes relative to the main beam, however, scales with the operational frequency. When the nine-element array is mechanically steered gimbaled to θ=10°, and is further electrically steered to between θ=9° and θ=10°, the sidelobes degradation is asymmetrical but with excellent far sidelobe rejection as the sidelobe degradation increases with offset angle. The same conditions can be applied to a 5 m diameter seven-element array. The sidelobe rejection of the hexagonal array is −13.5 dB below the peak gain of the main beam at a zero offset. Far sidelobe rejection for the nine-element array is −7 dB at a half beamwidth from the center and −4 dB at one beamwidth from the center. Far sidelobe rejection for the seven-element array is −8.8 dB at a half beamwidth from the center and −4.4 dB at one beamwidth from the center.
The nine and seven element arrays provide broadening main and sidelobe beamwidths with frequency as the angular positions of these beams changes and scales with frequency. Identical mechanical and electrical steering offers no degradation of sidelobe rejection, and there are no frequency dependent grating lobes. However, nonidentical mechanical and electrical steering injects asymmetrical sidelobe rejection degradation with good far sidelobe rejection. The sidelobe rejection of the nine-element rectangular array is −10 dB below the peak gain of the main beam at a zero offset. The sidelobe levels of the hexagonal array are −13.5 dB below the peak gain at a zero offset. Although the hexagonal array does offer improved performance of sidelobe suppression relative to the rectangular array, there are applications where sidelobe levels should be further reduced for improved performance. Hence, it has been desirable to provide an optimal packing density antenna array with good sidelobe rejection when both mechanical and electrical steering are at the same offsets. However, current antenna arrays only offer modest sidelobe rejection. These and other disadvantages are solved or reduced using the invention.